It depends on your code and the rest of the system. In some areas you need at least one full size vent through the roof. In other regions, you just need to have sufficient vents to equal the area of the main sewer.: : Terry -: I want to install at top of drain a 3" wye and below that a 3" combo. Down stream will be two lavarotories.: I want to vent off the top wye and reduce to 2".: My friend and part time plumber says that the vent is essentialy the soil stack and must remain 3".: He also says that I cannot lay a t on its back to drain the lavarotories into the sewer line.: I am confused can you help me.: thanks